1. Field of the Invention
The present invention relates to a transparent carbon nanotube (“CNT”) electrode comprising a conductive dispersant and a method for producing the same. More specifically, the present invention relates to a transparent CNT electrode comprising a transparent substrate and a CNT thin film formed on a surface of the transparent substrate wherein the CNT thin film contains carbon nanotubes (“CNTs”) and a doped dispersant, and a method for producing the transparent CNT electrode. The transparent CNT electrode of the present invention can be produced by a wet process, has excellent conductive properties, and can be applied to flexible displays.
2. Description of the Related Art
Transparent electrodes having a transparent substrate and a conductive film formed on the transparent substrate are now widely used to fabricate a variety of devices, including image sensors, solar cells, liquid crystal displays, organic electroluminescence (“EL”) displays and touch screen panels, that are required to have both light transmission properties and conductive properties.
Indium tin oxide (“ITO”) electrodes have been predominantly used as electrodes for transparent substrates because ITO shows a strong tendency to form a thin film on glass substrates and exhibits excellent light transmission properties and conductive properties. Vacuum deposition equipment is used to produce ITO electrodes. Particularly, sputtering equipment can be used in view of the excellent characteristics conveyed to the film by the sputtering technique. However, production of transparent electrodes by sputtering techniques requires a high processing temperature of 200° C. or higher, sometimes 400° C. or higher. Therefore, sputtering techniques are not suitable for the production of flexible displays that require transparent electrodes. Moreover, the use of the inflexible ITO electrodes in flexible displays causes increased sheet resistance and poor durability which is problematic for flexible displays.
To address these problems, extensive research is actively underway to investigate use of transparent electrodes based on carbon nanotubes (CNTs) as materials for conductive films formed on transparent substrates.
Carbon nanotubes (CNTs) are tubular materials made of carbon atoms in which one carbon atom is bonded to other adjacent carbon atoms in the form of a hexagonal-based honeycomb structure. CNTs are highly anisotropic, have various structures, such as single-walled, double-walled, multi-walled and rope (i.e., helical) structures, and have an extremely small diameters in the nanometer (1×10−9 meter) range. CNTs are known to have excellent mechanical properties, good electrical selectivity, superior field emission properties, highly efficient hydrogen storage properties, and the like. Particularly, CNTs can be advantageously used to form electrically conductive films due to their high electrical conductivity. CNTs can be synthesized by known methods including electrical discharge, pyrolysis, laser deposition, plasma chemical vapor deposition, thermal chemical vapor deposition, or electrolysis.
CNTs must be dispersed in suitable dispersion media in order to form conductive films. However, CNTs tend to aggregate by surface attraction, in particular by the intermolecular force referred to as Van der Waals attraction, where CNT's have a Van der Waals attraction of about 950 meV/nm. Since such aggregation of CNTs impedes the formation of three-dimensional networks that are capable of improving the mechanical strength and conductive properties of the CNTs, it is necessary to disperse the CNTs in suitable dispersion media.
Since most organic dispersants act as insulators, CNT thin films formed using organic dispersants generally exhibit poor conductive properties. Various efforts have been made to remove residual organic materials, which can act as insulators, from CNT films. For example, after a dispersion of CNTs and an organic material in water is used to form a CNT film, the organic material is removed from the film by dipping the film in water (Nano letters 2005, Vol. 5, No. 4, pp. 757-760). However, this method has difficulty in completely removing the organic material from the CNT film and ensuring reproducibility.
Further, although CNTs may be sufficiently dispersed in the conductive dispersant to form a conductive film, deterioration in the conductivity of the conductive film is inevitable because the conductive dispersant surrounds the surface of the CNTs and the conductivity of the conductive dispersant is much lower than that of the CNTs.